Cotton thrips, nematode interaction explored

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Cotton producers are too familiar with the potential damage from thrips and nematodes, and University of Georgia Extension specialists are now looking for possible interactions between these two pests.

“Anyone who grows cotton is familiar with thrips,” says Phillip Roberts, University of Georgia Extension entomologist. “When you think about insect pests in the Southeast, there’s no question they’re our most consistent pest.”

Growers are more than familiar with the visible damage caused by thrips, says Roberts, whether it’s the crinkling of leaves in certain plants, or more serious damage where no at-plant insecticides have been used, including stand loss or the loss of terminal dominance.

“We see a very consistent response to an at-plant treatment, and that’s why most of our growers use an at-plant treatment,” he says.

And it’s estimated, adds Roberts, that at least 70 percent of Georgia’s cotton acreage is infested with nematodes. “Many of our at-plant insecticides are used as at-plant treatments for nematodes. Temik, of course, has been the standard treatment of thrips and nematodes for many years. But we also have seed treatments that have activity on nematodes and thrips. When a farmer makes a decision about at-plant treatments, he’s thinking about both pests,” he says.

Whether a grower applies a nematicide in the furrow or as a fumigant, he’s basically treating a very small or limited area, he says. The life of the cotton plant and the half-life of the nematicide are relatively brief, so the effects are temporary.

“Basically, you have a zone where that seedling will be growing and developing that is perhaps protected from nematodes. But ultimately, nematodes will fill that zone back up. So you have a window to give that plant a head-start. We can get a good root zone established in 28 or 30 days, and then we can tolerate or outgrow a lot of potential problems with nematodes,” says Roberts.

One of the goals with a nematode management program is to have a rapidly developing cotton root system, says Roberts. “We’re familiar with how thrips will stunt cotton plants. We know the damage thrips can do, because we see severe stunting of above-ground plant growth. Have you ever thought about what is happening below ground?”

Researchers and Extension specialists participated in a project in 2006 looking at the interaction between pendimethalin injury and how the uptake of insecticides are impacted. In this research, thrips populations were correlated with root growth. Research also was conducted in California in the early 1990s looking at how thrips impact root growth.

The Georgia research, says Roberts, revealed an inverse relationship — the more thrips present, the less root development occurs. “For the last several years, we’ve been addressing this impact of thrips damage on roots in more detail,” he says.

One small plot trial showed that as thrips injury was decreased, root growth increased. “In that same trial, we looked at dry weights of the above-ground plant and dry weights of the roots, and there’s a strong correlation. What you see in terms of stunting above ground, the same type of stunting is occurring below ground,” he says.

This past year, researchers established treatments in pots, says Roberts, trying to get a variation in thrips injury from low damage to moderate damage to high damage. Roots were collected at two, three and four weeks after planting to see how growth was affected by the varying degree of thrips damage.

“It’s interesting that even three weeks after planting — we’re talking about two-leaf cotton — we’re already seeing differences in root growth when we haven’t done a good job of controlling thrips. At four weeks after planting, there are pretty distinct differences between the treatments that were used. When we looked at the root dry weight where we had our best thrips control, the root dry weight was about 50 percent more than where we had the seed treatment alone.”

It’s clear, he says, that thrips injury can impact root development, but does thrips management impact nematode control? Could inferior thrips control have an impact on how nematicides perform?

“Or, does superior thrips control enhance the plant’s ability to withstand nematodes, based on the premise or assumption that rapid early growth is an important part of nematode management?” asks Roberts.

Further research has been conducted in the past two years on a plot in Tifton, Ga., with a heavy infestation of nematodes. Main plots were treated with either at-plant insecticides or products that have activity on both insects and nematodes. Four-row plots were split, with half being treated with Orthene to minimize thrips injury and the other half was not treated. Thrips have been moderate to high in the plot during the past two years, says Roberts. Nematodes were in fairly high populations, but were very variable, he says.

“Basically, in 2008, there was not a statistical interaction. Looking at thrips damage, Temik provided more significant thrips control than seed treatments. We did see a significant reduction in thrips injury when we sprayed it with acephate.”

In 2009, using the same treatments, Temik again was the best performer in controlling thrips. The yield data from this past year did not show an interaction between treatments.

“That’s the main thing we’re looking for here — where those interactions exist,” says Roberts.

It’s very clear to see, he says, that thrips injury does impact root growth. “If you see above-ground stunting of seedlings, it’s also happening below the ground. At least in theory, and if you use common sense, about cotton growth and development in general, that root system is very important. And from a nematicide or nematode management standpoint, taking advantage of a nematode treatment during that window of opportunity is important.”

Roberts says he wants to emphasize he isn’t advocating that growers “go crazy” spraying for thrips in an effort to minimize injury. “There are a lot of other consequences associated with doing that, such as aphids and spider mites, and spider mites are a concern in Georgia.”

Research continues to show, he says, that it’s important to control thrips early in the life of the cotton seedling.

“Thrips control is important, and we don’t want to have to make a foliar application for thrips for several reasons. It’s important to select an effective at-plant insecticide that will provide the control needed.”